The invention relates to a directional illumination unit for autostereoscopic displays, which directs light which is emitted by activated illumination elements through a transmission-type image display means at visibility regions in the space in front of the display. After modulation of the light with image information and other information in the image display means, observer eyes can see a stereoscopic and/or monoscopic presentation.
The field of application of this invention includes autostereoscopic displays where the visibility regions can be tracked automatically to the eyes of multiple observers when they move to different positions in a relatively large viewing space in front of the display with the help of a tracking device and image controller. According to this invention, the images and other information can be provided to the observers optionally in a two-dimensional, in a three-dimensional or in a mixed mode.
An example of an autostereoscopic display with a directional illumination unit has been described by the applicant in the hitherto unpublished document DE 10 2006 042 325. Other special embodiments of such displays for sequential presentation of stereo images in synchronism with the formation of a visibility region for each observer eye have already been described in other applications and patents which are concerned with autostereoscopic displays and which have been filed by the applicant.
Seen in the direction of light propagation, the directional illumination unit of the autostereoscopic display according to this document comprises an illumination means with illumination elements which emit or transmit light, and an imaging means with imaging elements. The imaging elements image the light of activated illumination elements of the illumination means in almost parallel bundles of rays always onto one observer eye, where they form a visibility region. Each imaging element images a multitude of activated illumination elements, because the number of the regularly arranged illumination elements is larger by multiples than the number of the regularly arranged imaging elements.
For autostereoscopic presentation of images or other information there is the requirement that the entire area of both the visibility region and image display means must always be illuminated homogeneously in order to realise a high-quality 3D presentation. Further, cross-talking of the stereo images to the respective other eye shall be avoided.
The visibility region can be defined in various ways, and it can be large enough to serve either one or both eyes of an observer. Even if the observer moves to a new position in the viewing space in front of the display, they must be continuously provided with high-quality monoscopic and/or stereoscopic images.
If in an autostereoscopic display of the above-described type a stereoscopic representation is realised for two observers, for example, four visibility regions must be provided to serve the four different eye positions. In doing so, a number of problems must be solved in order to achieve a good imaging quality for a large number of observers.
First and foremost, in the interest of an acceptable user convenience, the visibility regions for the two eyes of one observer must be provided somewhat distant to the visibility regions for the two eyes of another observer, so to circumvent mutual interfering of the observers in front of the autostereoscopic display. However, the optical imaging conditions for an observer are optimal when their eyes are situated in a central position in front of the display, i.e. near its optical axis.
As the lateral distance of the observer eyes from the centre of the display becomes larger, the image quality deteriorates because the emission angles of the bundles of rays become larger. If a lenticular is used as an imaging means, for example, optical aberrations will occur which may impede the creation of sufficiently large defined visibility regions. Further, the aberrations do not allow large viewing angles, so that the light beams do not reach an observer who is situated too far away from the central axis of the display. The viewing angle or viewing space is here defined as a space or range in front of the display, seen from the observer's perspective, where observers can be situated and watch a presentation in the desired mode. However, a large viewing angle is essential in particular for a multi-use display. The above-mentioned disadvantages can be circumvented by using a controllable deflection unit.
A controllable electro-optical cell, a so-called electrowetting cell, is known from the international publication WO 2004/099847. These cells take advantage of the capillary effect and electrowetting effect in order to modify the surface tension of liquids using electrostatic potential and so to control the optical refraction behaviour. An electrowetting cell basically comprises a capacitor which is filled between the electrodes with a hydrophobic liquid, such as an oil, and water, where one of the electrodes is coated with a hydrophobic material. Without an electric field being applied, the oil covers the coated electrode as a film, and with an electric field being applied, the water displaces the oil film, because the applied field compensates the polarisation of the dipoles in the water surface. There is an interface between the two materials. The cell can be controlled electronically to realise optical lenses and prism elements with a surface area of less than one square millimeter.
Due to the variable control of the inclination of the interface in an electrowetting cell, the optical refraction behaviour can be changed continuously, which is why optical aberrations can be reduced much better than with conventional optical elements. Thanks to these properties, electrowetting cells are optimally suited for usage in devices which realise optical and wave-optical functions.
An autostereoscopic display device according to the international publication WO 2004/075526 A2 takes advantage of such an electrowetting cell. In that display, collimated light is transmitted through a flat display means with discrete pixels, which generates stereoscopic video images for each observer eye which differ in parallax.
The flat display means comprises a beam splitter and a dynamically controllable beam controller with electrowetting cells in order to direct the video images at the respective eye of observers. In the controllable beam controller, each modulator pixel of the display means is fixedly assigned to an electrowetting cell, so to adjust the light exit angle of the light beams whose intensity has been modified such that it is directed at the current eye position. The fixed assignment of the electrowetting cells and the pixels of the flat display panel requires great technological effort in particular as regards the manufacturing precision. That system is designed for point-wise modification of incoherent light and makes no demands whatsoever on interference conditions. In particular the non-linear transmission behaviour in the marginal zones of the electrowetting cells would affect the propagation of the modulated light and thus reduce the quality of the 3D representation if electrowetting cells and pixels are insufficiently aligned.
This invention shall widely overcome the disadvantages described in the prior art section.
It is therefore the object of the present invention to provide for light guidance in an above-described autostereoscopic display with simple optical means a directional illumination unit which allows multiple observers to watch a 2D and/or 3D presentation without any disturbance simultaneously from respectively assigned visibility regions in a viewing space in front of the display. The space in front of the display which is accessible for a tracking and image controller shall therefore be relatively large. The directional illumination unit according to this application shall transmit the light which is emitted by the illumination means into the visibility region widely loss-free and homogeneous.
Further, an assignment of an electrowetting cell to a pixel of the fine pixel structure of the image display means shall be avoided. Further, it shall be avoided that the directional illumination unit adversely affects the light which modulated by the image display means during its further propagation. Further, an improved image quality of the autostereoscopic display shall be achieved at the same time compared with prior art devices thanks to the arrangement of the optical means.